NATURAL SOURCE MAGNETOTELLURICS
May 2002 Section 13, Page 2
13.1 INTRODUCTION
Natural source magnetotellurics (MT) or audio-frequency magnetotellurics (AMT) is a frequency
domain EM program that uses naturally occurring random noise as the signal source. This data
acquisition system uses cascade decimation and stacking and averaging of Fourier transformed
cross and auto-power spectra of the 6th and 8th harmonics, to obtain amplitude and phase
measurements of the electric and magnetic fields.
The frequency range of the MT/AMT program is from 0.0007 (6/8192) to 8192 Hz, and is
divided into 4 groups as shown below, with the 6th and 8th harmonics displayed:
Data for the low band are acquired on a continuous basis, with the filtering, decimation and
Fourier transforms being done real-time. Data are acquired for the upper three bands in a "burst"
mode with data processing being done between bursts.
Data are accepted or rejected according to coherency and outlier limit tests. The process is
explained in more detail in the menu discussions, which follow.
Both electric field (Ex, Ey) measurements and magnetic field (Hx, Hy, Hz) measurements are
utilized with this method. A calibrate buffer is provided for the magnetic antenna calibrates, and
is labeled 8) AMT Antenna Cal in the calibrate buffer area. The standard board calibrate
buffer is labeled 7) AMT Calibrate.
This manual is written in generic form for a 16-channel receiver. If your receiver has less than
16 channels, only information for the number of channels contained will be displayed.
Go to Section 6 for information concerning calibration, synchronization and generic operation of
all programs.
Go to the end of this AMT program manual for suggestions on connecting the receiver for field
measurements.
Low band High band Very high band
(SR = 16 Hz) (SR = 4096 Hz) (SR = 32768 Hz)
3. Hz 4. Hz 768 1024 6144 8192
1.5 2. 384 512 3072 4096
.750 1. 192 256 1536 2048
.375 .5 96 128 768 1024
.1875 .250 48 64 384 512
.09375 .125
.046875 .0625 Medium band
.023237 .03125 (SR = 256 Hz)
.0117188 .015625 48 64
.0058538 .0078125 24 32
.0029297 .0039063 12 16
.0014648 .0019531 6 8
.0007324 .0009766 3 4
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